Try turning a growing plant on end, and a funny thing happens. It will right itself, twisting and contorting so its leaves project skyward and its roots extend down toward the center of the Earth. Logic would dictate that plants solve the problem of which way to grow by sensing gravity — but how did scientists ever prove that this is correct?

Over on NPR, Robert Krulwich recounts the story of scientist Thomas Knight and the first experiment to demonstrates that plants do, in fact, sense gravity:

He attached a bunch of plant seedlings onto a disc (think of a 78 rpm record made of wood). The plate was then turned by a water wheel powered by a local stream, "at a nauseating speed of 150 revolutions per minute for several days."

If you've ever been at amusement park in a spinning tea cup, you know that because of centrifugal force you get pushed away from the center of the spinning object toward the outside.

Knight wondered, would the plants respond to the centrifugal pull of gravity and point their roots to the outside of the spinning plate? When he looked... that's what they'd done. Every plant on the disc had responded to the pull of gravity, and pointed its roots to the outside. The roots pointed out, the shoots pointed in. So Thomas Knight proved that plants can and do sense gravitational pull.

Problem solved! Sort of. As is wont to happen with good, interesting science, Knight's experiments simply led to more questions. What is it, exactly that makes it possible for plants to sense gravity in the first place?